STANDING TALL: Jason Collins, who began his NBA career with the Nets and spent time this season with the Celtics, yesterday became the first active male athlete in a major U.S. sport to come out as gay.Getty Images

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BOSTON — In a ground-breaking announcement, Jason Collins, a 12-year NBA veteran who began his career with the Nets, has come out as the first openly gay active male athlete in an American team sport.

“I’m a 34-year-old NBA center. I’m black. And I’m gay,” Collins revealed in a lengthy article he wrote for the May 6 edition of Sports Illustrated.

“I didn’t set out to be the first openly gay athlete playing in a major American team sport. But since I am, I’m happy to start the conversation.”

Almost immediately, the social media world was inundated with messages, overwhelmingly supportive of Collins, after the story appeared online yesterday.

“Don’t suffocate who u r because of the ignorance of others” Kobe Bryant wrote on Twitter.

President Obama reached out by phone. Former president Bill Clinton, NBA Commissioner David Stern, the NBA Players Association and the Wizards, the team for which Collins last played, offered support. Collins, a Net for 6 1/2 seasons, is headed to free agency this summer.

Former and current members of the Nets had nothing but positives to say about Collins, though some admitted shock at the revelation.

“I didn’t know. Honestly, I was shocked,” former teammate Kerry Kittles told The Post. “But I’m happy for him. Jason Collins is one of the classiest guys I’ve ever known. I could give a [hoot] about his sexual orientation. He really is one of the best teammates I ever played with.”

Rod Thorn, who was president and general manager during Collins’ time with the Nets, voiced similar sentiments. He had no idea of Collins’ orientation, which the center said in the article he went to great lengths to mask.

“It shocked me,” Thorn said. “During the time he was with us, he had a girlfriend … and I thought they were very serious. But I’m happy for Jason. He was a terrific player, a smart player, a team-first player for us. He was a tough, physical guy.”

Ed Stefanski, a former Nets general manager now with Toronto, also expressed positive feelings for Collins, whose twin brother Jarron also played in the NBA.

“I texted Jason because I’m so happy for him that the weight is off his shoulders,” Stefanski said. “I’m happy if Twin’s happy.”

Two current Nets, Joe Johnson, a teammate of Collins’ in Atlanta, and Brook Lopez, like Collins a Stanford center with a twin brother in the NBA, released statements.

“Jason Collins was one of the best teammates I ever had,” Johnson said. “I respect his tremendous courage to come out and will always support him.”

Lopez said, “It is an honor for me to call Jason Collins a friend. I admire his dignity as well as his courage to come out. I will always have his back.”

Collins said in the article that he did not tell Jarron until last summer. In a companion article, Jarron Collins wrote, “I won’t lie. I had no idea. … Jason has taken a huge weight off his shoulders. And I’ve never been more proud of him.”

Jason Collins wrote he finally decided to come out after the tragedy of the Boston Marathon brought the realization that life can be horribly altered in a flash.

“Things can change in an instant, so why not live truthfully?” Collins wrote.

Collins said he dated women because he felt he had to. But he knew he was deluding himself as well as others.

“No one wants to live in fear,” he wrote.

Collins is not the first male athlete to come out — but he is first to do so while still active in one of the four major leagues. Former NBA center John Amaechi admitted he was gay after his retirement. Recently, Baylor’s Brittney Griner, headed to the WNBA, admitted she is gay.